Electro-mechanical transducer with negative feedback for the recording and reproduction of sound waves



May 12, 1964 s. NEUMANN 3,133,161 ELECTRO-MECHANICAL TRANSDUCER WITH NEGATIVE FEEDBACK FOR THE RECORDING AND REPRODUCTION OF SOUND WAVES Filed Jan. 30, 1961 2 sheets-sheet 1 l u I x I a N 13 I 6 l K Fig. 2

Jn venzor:

May 12. 1964 33,161

G. NEUMANN 3,1 ELECTRO-MECHANICAL TRANSDUCER WITH NEGATIVE FEEDBACK FOR THE) RECORDING AND REPRODUCTION OF SOUND WAVES Filed Jan. 30, 1961 2 Sheets-Sheet 2 Fig- 3 Jnvenfor:

United States Patent ELECTRO-MECHANICAL TRANSDUCER WITH NEGATIVE FEEDBACK FOR THE RECORDING AND REPRODUCTION OF SOUND WAVES Georg Neumann, 2 Segitzdamm, Berlin, Germany Filed Jan. 30, 1961, Ser. No. 85,812 Claims priority, application Germany Jan. 30, 1960 Claims. (Cl. 179-100.41)

The present invention relates to an electro-mechanical transducer with negative feedback, operating in accordance with the electro-dynamic principle, for the recording and reproduction of sound waves in lateral recording technique for instance, for cutting of disk records, on which the sound recording is performed mechanically in grooves.

Electro-dynarnic one-component disc recorders are known, in which the stylus is secured to the tip of an oscillating body consisting of a movable cone, which is equipped with a driving coil and a negative-feedback-coil. The driving coil is disposed therein in the vertical air slot of a magnetic circuit, while the negative-feedbackcoil is arranged in a horizontal air slot .of a secondary magnetic circuit, both coils having a common axis. The oscillating body is rotatably suspended by means of a flat spring around a horizontal axis, disposed between the driving coil and the negative-feedback-coil. In the known arrangement, the center of gravity of the oscillating body is arranged in the axis of the mentioned flat spring. A voltage being in phase opposition to the current in the driving coil is induced in the negative-feedback-coil, which voltage is proportional to the mechanical vibrations of the oscillating body. This negative-feedback-voltage is fed to the driving coil, thereby linearizing the frequency response of the system and compensating distortions which may occur.

In the known arrangement, undesirable resonances of the :oscillating body occur already at the upper limit of the transducers-frequency range. The distance of these resonances from the upper end of the frequency range must be, however, comparatively great, since already prior to the reaching of an undesirable resonance, phase shifts occur between the driving and the feedback-voltage. The negative-feedback thereby, in these ranges will become a positive feedback and thus leads easily to the selfexcitation of the system. The effective range of the negative-feedback remains, thereby, small. No negativefeedback exists anymore at the upper and lower limits of the frequency range, so that the distortions cannot be reconciled anymore.

The fiat spring, used in the known arrangement for the suspension of the oscillating body, has the drawback that undesirable vibrations parallel to the axis of the oscillat ing body occur, though the latter is supposed to perform only rotary vibrations.

A two-component recorder with negative-feedback is also known, wherein the oscillating body has the shape of a rotary-symmetrical hollow body, which is tapered in downward direction, and at the apex of which the cutting stylus is disposed. The driving coils for the two components are disposed at the edge and two negativefeedback coils are arranged in the vicinity of the apex of the hollow body.

The suspension of the oscillating body by means of an axle has appreciable disadvantages in this two-component recorder, since this suspension permits of resonances,

which are still within its frequency range. Furthermore, the driving coils and the feedback coils are disposed on the same, that means, in two planes arranged parallel to each other. Thus the driving coil and the feedback coils must be arranged spaced apart from each other-to such an extent that a sufliciently large electrical decoupling results. The building height of the oscillating body can thus not be chosen arbitrarily small, rather it is necessary to take into consideration the required decoupling between the driving coil and the feedback coils.

It is, therefore, one object of the present invention to provide an electro-mechanical transducer with means for negative feedback for the recording and reproduction of sound waves, wherein the oscillating body is a conically shaped hollow body and the driving coil is arranged on the cone-shaped shell, which has at this point such inclination, that the forces of the driving coil are directed towards the apex of the recording stylus, while one or two feedback coils are disposed rectangularly to the plane of the driving coil. The oscillating body is suspended by means of a membrane arranged inside said body.

The membrane, suspended in the hollow body, is, for instance, clamped between two distancing disks by means of two bolts at two points, so that the oscillating body can swing about a horizontal axis constituted by the connecting line of these two points. This membrane suspension has the advantage, that the membrane is extremely elastic in the direction of the wanted movement, yet in all other directions, but particularly rectangularly to the direction of the wanted movement, so rigid that disturbing cross-vibrations occur only in a freguency range, which lies far outside of the frequency range of the transducer.

It is another object of the present invention to provide an electro-mechanical transducer with negative feedhack for the recording and reproduction of sound waves, where in the mounting of the oscillating body, by means of the membrane secures movement of said body around the same axis, around 'which the body would swing under the influence of the driving forces even without being supported. This axis is arrived at from the mass and the movement of inertia of the oscillating body, the position of its center of gravity and the position of the point of application of the resulting driving forces. Depending upon the prevailing circumstances, the rotary point may be arranged above or below the center of gravity. The mounting of the oscillating body at that point has the advantage that movements of the body are caused by the rotary forces only and not upon any other forces. In this manner a load by undesirable forces, particularly by transverse forces, is impossible and a deflection of the oscillating body in undesirable directions cannot occur;

The driving coil is disposed above the membrane out-. side of the oscillating body. A cylindrical coil or a conical coil can be applied. It is, however, advantageous to choose the inclination of the coil such that the forces, exerted by the driving coil, meet at the apex of the recording stylus. Inthis manner it is possible to achieve additionally that the driving forces being applied to the oscillating body are effective nearly inthe direction of its greatest rigidity. Thus the occurrence of disturbing adthe best possible decoupling is achieved. The two feed back coils are preferably flat and of circular shape and electrically connected in such manner that disturbing voltages induced by outer magnetic fields will compensate each other. Thus suitably two feedback coils are used, though under certain circumstances one feedback coil suflices.

With these and other objects in view, which will be come apparent in the following detailed description, the present invention will be clearly understood in connection with the accompanying drawings, in which:

FIGURE 1 is a longitudinal section through the swinging body along the symmetrical axis;

FIG. 2 is a schematic plan View of the membrane for suspending the swinging body; and

FIG. 3 is a schematic plan view indicating the arrangement of the negative-feedback coils in the magnetic field.

Referring now to the drawings, the present device comprises a base plate 1 made of non-magnetic material to which the magnet-system (not shown) is secured. Merely the pole-shoe 2, the soft-iron core 3 and the pole-shoe 4 are shown, which elements define a conically shaped air slot, in which the driving coil 5 swings. The coneshaped oscillating body is divided into an annular, conical upper portion 6, on which the mentioned driving coil 5 is secured, and into a lower portion 7, which carries at its apex the recording stylus S and at its upper edge the membrane 9. The securing of the membrane 9, and thereby, of the entire oscillating body, with the soft-iron core 3, is made by means of a massive part with two bolts 11. The membrane 9 is clamped to the two bolts 11 by using distancing disks 12.

If the conical ring 6 consists of metal, the driving coil 5 produces in the conical ring 6 an additional shortcircuit-current, which has no phase coincidence with the current in the driving coil. The effect of the short-circuitcurrent increases with increasing frequency, beginning at 2000-3000 Hz. The movement of the oscillating body has thus a phase shifting in relation to the driving current, so that the voltage, induced in the feedback coils, has a phase shifting in relation to the driving current. The phase of the feedback-voltage is thus turned for more than 90, so that the negative-feedback passes over into a positive feedback.

In order to avoid this drawback, the annular, conical portion 6 is equipped with a slot 13, so that no shortcircuit-currents can be formed. In order not to shortcircuit the portion 6 again by the portion 7, both parts are electrically separated by means of an intermediate layer, for instance, by means of a lacquer, though both parts are mechanically rigidly connected with each other.

The two feedback coils are fiat and of circular shape and they have thus the greatest possible rigidity in the direction of the swinging. The two coils 14 and 15 are emerged into a corresponding slot, disposed between the auxiliary pole-shoes 16 and 17, which are magnetically connected with the pole-shoes 2 and 4 for the driving coil 5. The arrows, shown in the drawings, indicate the movement of the feedback coils, following the movement of the oscillating body in the direction of the arrow shown in the drawing next to the record ng stylus.

As much as possible non-magentic material is used for the construction of the transducer, only the coil 3 and the magnet with the pole-shoes 2 and 4, as well as the auxiliary pole-shoes 16 and 17, respectively, consist of iron. For instance, bronze is used for the membrane 9.

The transducer, described above and designed in accordance with the present invention, has the advantage that it is of simple structure and, thereby, best suitable for the manufacture thereof in large quantities. A nonobjectionable and reliable centering of the oscillating body is obtained by the membrane suspension.

The transducer designed in accordance with the present invention can be applied not only for a one-channel recording in lateral disk-recording technique, but also for all recordings in accordance with the light-sound process. Light-sound recordings may be performed with the present transducer Without any difficulty, if a mirror is used instead of the recording stylus, which mirror declines the received light ray according to the applied audio-frequency voltages.

While I have disclosed one embodiment of the present invention, it is to be understood that these embodiments are given by example only and not in a limiting sense, the scope of the present invention being determined by the objects and the claims.

I claim:

1. An electro-mechanical transducer operating in accordance with the electro-dynamic principle for recording and reproduction of sound waves in a lateral recording technique comprising an oscillating body having a cone-shaped shell, including a rotary-symmetrical hollow cone tapered in downward direction,

a stylus secured to the apex of said oscillating body,

a permanent magnet surrounding said oscillating body and forming pole-shoes to create a magnetic field,

a driving coil disposed at the periphery of said oscillating body,

at least one feed-back coil adjacent to the apex of said oscillating body,

the improvement residing in said driving coil being disposed on said cone-shaped shell of said oscillating body,

said cone-shaped shell being inclined such that its surface and thereby the magnetic forces emerging from said driving coil and from said permanent magnetic field being directed towards the apex of said stylus,

said feedback coil being disposed rectangularly to the plane of said driving coil,

a soft-iron core immovably disposed within a portion of said oscillating body, and

a membrane mounted on said soft-iron core and disposed inside of and supporting said oscillating body.

2. The device, as set forth in claim 1, which includes a base plate, two bolts projecting from said base plate, and said bolts including means for rigidly clamping said membrane to said bolts, in order to provide a two-point suspension for said membrane.

3. The device, as set forth in claim 2, wherein the axis of rotation of said oscillating body coincides with the axis about which said oscillating body would move without said suspension, upon driving said oscillating body by a horizontally directed force at the tip of said stylus by two magnetic forces from both sides of said driving coil, said two magnetic forces being combined to one horizontally directed force at the tip of said stylus.

4. The device as set forth in claim 2, wherein said clamping means comprises two spacing disks threadedly ecured to said bolts, and said membrane is clamped between said spacing disks.

5. The device, as set forth in claim 2, wherein said oscillating body comprises an annular, conical upper portion carrying said driving coil, and a conical lower portion carrying at its bottom said recording stylus and at its top the marginal zone of said membrane.

6. The device, as set forth in claim 5, which includes a soft-iron core disposed in said upper portion of said oscillating body, a massive part of non-magnetic material disposed in said lower portion of said oscillating body, and said bolts are secured to said massive part, so that said membrane with said oscillating body is secured to said oft-iron core by means of said massive part and said bolts.

7. The device, as set forth in claim 5, wherein said conical upper portion of said oscillating body has a slot disposed in a plane extending through the longitudinal axis of said oscillating body.

8. The device, as set forth in claim 5, which includes an electrically insulating layer disposed between said upper portion and said lower portion of said oscillating body.

9. The device, as set forth in claim 1, wherein said feedback coil is fiat and of circular shape and is electrically connected in a manner, that disturbing voltages, induced by outer fields, will compensate each other.

10. The device, as set forth in claim 1, which includes auxiliary pole-shoes forming at least one slot, and said feedback coil is immersed in said slots of said pole-shoes.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ELECTRO-MECHANICAL TRANSDUCER OPERATING IN ACCORDANCE WITH THE ELECTRO-DYNAMIC PRINCIPLE FOR RECORDING AND REPRODUCTION OF SOUND WAVES IN A LATERAL RECORDING TECHNIQUE COMPRISING AN OSCILLATING BODY HAVING A CONE-SHAPED SHELL, INCLUDING A ROTARY-SYMMETRICAL HOLLOW CONE TAPERED IN DOWNWARD DIRECTION, A STYLUS SECURED TO THE APEX OF SAID OSCILLATING BODY, A PERMANENT MAGNET SURROUNDING SAID OSCILLATING BODY AND FORMING POLE-SHOES TO CREATE A MAGNETIC FIELD, A DRIVING COIL DISPOSED AT THE PERIPHERY OF SAID OSCILLATING BODY, AT LEAST ONE FEED-BACK COIL ADJACENT TO THE APEX OF SAID OSCILLATING BODY, THE IMPROVEMENT RESIDING IN SAID DRIVING COIL BEING DISPOSED ON SAID CONE-SHAPED SHELL OF SAID OSCILLATING BODY, SAID CONE-SHAPED SHELL BEING INCLINED SUCH THAT ITS SURFACE AND THEREBY THE MAGNETIC FORCES EMERGING FROM SAID DRIVING COIL AND FROM SAID PERMANENT MAGNETIC FIELD BEING DIRECTED TOWARDS THE APEX OF SAID STYLUS, SAID FEEDBACK COIL BEING DISPOSED RECTANGULARLY TO THE PLANE OF SAID DRIVING COIL, A SOFT-IRON CORE IMMOVABLY DISPOSED WITHIN A PORTION OF SAID OSCILLATING BODY, AND A MEMBRANE MOUNTED ON SAID SOFT-IRON CORE AND DISPOSED INSIDE OF AND SUPPORTING SAID OSCILLATING BODY. 